Absorption/reduction type catalyst for nox removal

ABSTRACT

A catalyst able to overcome defects of an absorption reduction-type NO x  purifying catalyst, such as poor NO x  purifying capability at low temperatures and low SO x  desorbing property, is provided.  
     The catalyst is an absorption reduction-type NO x  purifying catalyst where NO x  absorbent particles and support particles having supported thereon a catalyst component are mixed. Preferably, acidic support particles are added to the support particle, the NO x  absorbent particles are a metal oxide having a base point, and the metal oxide is rare earth-added zirconia. In conventional catalysts, the NO x  absorbent such as alkali metal having strong basicity exposes the catalyst component present together on the same support to the basic condition to decrease the catalytic performance. However, in the present invention, the NO x  absorbent and the support are separate particles, whereby the catalyst component can exert its original catalytic performance and, as a result, the NO x  purifying capability at low temperatures and the SO x  desorbing property are improved.

TECHNICAL FIELD

[0001] The present invention relates to an exhaust gas purifyingcatalyst for purifying an exhaust gas from an internal combustion engineof automobiles and the like and, more specifically, the presentinvention relates to an absorption reduction-type NO_(x) purifyingcatalyst improved in a NO_(x) purifying capability.

BACKGROUND ART

[0002] Recently, from the standpoint of global conservation, it is aworldwide problem to suppress the total amount of carbon dioxide (CO₂)exhausted from an internal combustion engine such as an automobileengine and the amount of nitrogen oxide (NO_(x)) generated. In order tosolve this problem, a lean-burn engine has been developed for thepurpose of improving the fuel consumption and an absorptionreduction-type NO_(x) purifying catalyst, obtained by adding a functionof absorbing NO_(x) in a lean atmosphere to a conventional three-waycatalyst, has been developed for the purpose of purifying the exhaustgas of the lean-burn engine. These are making certain progress insolving the above-described problems.

[0003] In the lean-burn engine combined with the absorptionreduction-type NO_(x) purifying catalyst, the fuel is usually burned atan air-fuel ratio in the lean (oxygen-excess) condition and temporarilyburned in a stoichiometric (at a theoretical air-fuel ratio) or rich(fuel-excess) condition.

[0004] HC (hydrocarbon) or CO in the exhaust gas is efficiently burnedand removed in the lean condition by the action of catalyst due to theoxidative atmosphere. On the other hand, NO_(x) is captured by anabsorbent in the lean condition and this is temporarily released in thestoichiometric or rich condition and reduced and purified by the actionof a catalyst due to the reducing atmosphere.

[0005] By virtue of these combustion conditions and the action of theabsorption reduction-type NO_(x) purifying catalyst, as a whole, thefuel consumption is improved and at the same time, HC, CO and NO_(x) inthe exhaust gas can be purified with good efficiency.

[0006] In this absorption reduction-type NO_(x) purifying catalyst, anoble metal such as platinum, gold, palladium and rhodium is used as thecatalyst component and a basic substance such as alkali metal (e.g.,potassium, sodium) and alkaline earth metal (e.g., calcium, barium) isused as the NO_(x) absorbent.

[0007] This lean-burn system established by combining the control of anair-fuel ratio and the NO_(x) absorbent is successful to a certainextent in solving the problem to improve the fuel consumption and reducethe total generation amount of CO, HC and NO_(x) as compared with theconventional exhaust gas purifying system using a three-way catalyst anda nearly theoretical air-fuel ratio.

[0008] The techniques on this absorption reduction-type NO_(x) purifyingcatalyst are described in Japanese Unexamined Patent Publication (Kokai)Nos. 7-51544, 7-136514, 9-24247 and 11-14422, filed by the presentapplicant, or the like.

[0009] In any absorption reduction-type NO_(x) purifying catalyst ofthese prior techniques, an alkaline earth metal is used as the NO_(x)absorbent and the catalyst component such as platinum and the NO_(x)absorbent both are supported on a support such as γ-alumina.

[0010] However, these absorption reduction-type NO_(x) purifyingcatalysts of conventional techniques have a problem that the NO_(x)purifying capability is poor when the exhaust gas temperature is low andabout 300° C. or less. In addition, SO_(x), of which source is sulfurcontained in a slight amount, forms a salt with the NO_(x) absorbentduring the combustion and this SO_(x) is not easily desorbed from theNO_(x) absorbent. As a result, the NO_(x) purifying capabilitydisadvantageously decreases over time.

[0011] As such, conventional absorption reduction-type NO_(x) purifyingcatalysts are in need of improvement in both the NO_(x) purifyingcapability at low temperatures and the SO_(x) desorbing property.Particularly, in order to apply the catalyst to a low-temperatureexhaust gas containing a relatively large amount of SO_(x), such asdiesel engine exhaust gas, those capabilities must be greatly improved.

[0012] Accordingly, an object of the present invention is to provide acatalyst having a different structure from conventional techniques andthereby provide an exhaust gas purifying catalyst freed from theabove-described problems and capable of exhibiting a high NO_(x)purifying capability at low temperatures and an improved SO_(x)desorbing property.

DISCLOSURE OF THE INVENTION

[0013] The object of the present invention can be attained by anabsorption reduction-type NO_(x) purifying catalyst where NO_(x)absorbent particles and support particles having supported thereon acatalyst component are mixed.

[0014] More specifically, the exhaust gas purifying catalyst of thepresent invention is a catalyst where the NO_(x) absorbent particles andthe support particles having supported thereon a catalyst component areseparate particles and these support particles and absorbent particlesare mixed. NO_(x) is preferably taken in by absorption to the surface orinside of the NO_(x) absorbent particles, while keeping its chemicalstructure substantially in an intact state.

[0015] In the present invention, the NO_(x) purifying capability at lowtemperatures and the SO_(x) desorbing property are remarkably improvedby using the support and the NO_(x) absorbent as separate particles. Thereasons therefor are thought to be as follows.

[0016] In conventional absorption reduction-type NO_(x) purifyingcatalysts, the NO_(x) absorbent is an alkali metal or an alkaline earthmetal as described above and such a metal shows strong basicity. If thecatalyst component and the NO_(x) absorbent are present together on thesame support, the NO_(x) absorbent electrically acts on the catalystcomponent through the support and, as a result, the performance of thecatalyst component is decreased.

[0017] Particularly, the catalyst component such as platinum decreasesin the oxidizing capability of NO NO₂ and the HC oxidizing capabilityunder the lean condition and, thereby, the capability of absorbingNO_(x) at low temperatures and the HC purification percentage aredecreased. Furthermore, the NO_(x) purification percentage also does notincrease at a high temperature.

[0018] On the other hand, when the NO_(x) absorbent and the support areseparate particles as in the present invention, the NO_(x) absorbentdoes not electrically act on the catalyst component and, therefore, thecatalyst component can exert its original catalyst performance, wherebythe NO_(x) purifying capability at low temperatures and the SO_(x)desorbing property both are improved as compared with conventionalcatalysts using an NO_(x) absorbent such as alkali metal.

[0019] Moreover, conventional NO_(x) absorbents such as alkali metalabsorb NO_(x) or SO_(x) in the form of a nitrate or a sulfate andsufficient absorption is attained when such a salt is formed even to theinside of the NO_(x) absorbent particle, however, the speed at which thesalt is formed even to the inside and the speed at which NO_(x) or thelike is released from the internally formed and firmly bonded salt arelow and this gives rise to low efficiency in the absorption/release ofNO_(x) or the like.

[0020] On the other hand, in the present invention, the NO_(x) absorbentis particles separate from the support and is not restricted by theamount of the absorbent supported on the support as in conventionalNO_(x) absorbents such as an alkali metal. Therefore, in the presentinvention, the NO_(x) absorbent may be used in a relatively large amountso as to enable sufficient NO_(x) purification even when NO_(x) isabsorbed only on the surface of the NO_(x) absorbent or in the vicinityof the surface, or an absorbent for absorbing NO_(x) or the like throughrelatively weak bonding may be used to overcome the low efficiency inthe absorption/release of NO_(x) or the like, whereby the NO_(x)purifying capability at low temperatures and the SO_(x) releasabilitycan be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a model view showing one embodiment of the catalyststructure of the present invention.

[0022]FIG. 2 is a model view showing the catalyst structure of aconventional technique.

[0023]FIG. 3 is a model view showing another embodiment of the catalyststructure of the present invention.

[0024]FIG. 4 is a model view showing another embodiment of the catalyststructure of the present invention.

[0025]FIG. 5 is a graph comparing the NO oxidizing capability ofcatalysts.

[0026]FIG. 6 is a graph comparing the SO_(x) desorbing property ofcatalysts.

BEST MODE FOR CARRYING OUT THE INVENTION

[0027] The absorption reduction-type NO_(x) purifying catalyst of thepresent invention is composed of NO_(x) absorbent particles and supportparticles having supported thereon a catalyst component.

[0028] As the catalyst component, a noble metal such as platinum, gold,palladium and rhodium can be used.

[0029] As the NO_(x) absorbent particles, particles which can take inmainly NO_(x) between lattices or into vacancies or tunnels or which canabsorb NO_(x) on the surface or inside of the particles whilesubstantially keeping the chemical structure intact, for example, byforming a solid solution with NO_(x), can be used. Here, NO_(x) maypartially form a salt with the absorbent particle or the salt maydissolve in the NO_(x) absorbent.

[0030] In a preferred embodiment, the NO_(x) absorbent is a metal oxidehaving a base point for donating an electron, namely, a metal oxide fordonating an electron to NO_(x) to form a negative ion and capturingNO_(x) by the electrical action between the negative ion and thepositive electric charge at the portion from which the electron isreleased. Examples of such a metal oxide include rare earth-addedzirconia having an oxygen defect, such as La_(x)Zr_(1-x)O_((2-x/2))(x=0.01 to 0.70), and alkaline earth-added zirconia having an oxygendefect, such as Sr_(x)Zr_(1-x)O_((2-x)) (x=0.01 to 0.50).

[0031] As the support particle, a material having a high specificsurface area and a fine form is suitably selected from alumina, silica,titania, zirconia, ceria and the like so as to provide a wide contactarea with the exhaust gas.

[0032] In a preferred embodiment, an acidic support particle is added tothe above-described support particle. Examples of the acidic supportparticle include WO₃/ZrO₂, alumina-silica, and zeolite. By thisaddition, the NO_(x) purifying capability at low temperatures and theSO_(x) desorbing property can be more improved. This is considered tooccur because although the catalyst such as platinum originally exertsthe catalytic activity in the acidic side as described above, an acidiccondition is formed by the addition of the acidic support particle andmoreover, NO_(x) and SO_(x) move fast on the acidic support.

[0033] Also, in a preferred embodiment, the support particles are formedby adding WO₃/ZrO₂ or the like to powder particles having a highspecific surface area, such as γ-alumina. This is because γ-aluminahaving a high specific surface area is easily available but WO₃/ZrO₂itself having a high specific surface area is difficult to obtain.

[0034] The WO₃/ZrO₂ is, as shown in Examples later, a particle obtainedby depositing WO₃ on the surface of ZrO₂ particles.

[0035] The catalyst of the present invention comprising these catalystcomponent, support particles and NO_(x) absorbent particles can beobtained by mixing the support particles and the NO_(x) absorbentparticles using a general method to form a slurry, coating the slurry ona monolith substrate, drying and calcining the slurry, then loading thecatalyst component thereon, and drying and calcining it. The catalyst ofthe present invention can also be obtained by mixing the supportparticles having supported thereon the catalyst component with theNO_(x) absorbent particles to form a slurry, coating the slurry on amonolith substrate, and drying and calcining the slurry.

[0036] The catalyst component can be loaded, for example, by deposition,precipitation, adsorption or ion exchange.

[0037] FIGS. 1 to 4 show some embodiments of the absorptionreduction-type NO_(x) purifying catalyst of the present inventiontogether with the catalyst structure of a conventional technique.

[0038]FIG. 1 shows the state where a noble metal as the catalystcomponent is supported on γ-alumina as the support particles and theNO_(x) absorbent particles are present as particles separate from thesupport particles.

[0039]FIG. 2 shows an absorption reduction-type NO_(x) purifyingcatalyst of a conventional technique, where both the NO_(x) absorbentand the noble metal as the catalyst component are supported on γ-aluminaas the support particle.

[0040]FIG. 3 shows the state where platinum as the catalyst component issupported on γ-alumina as the support particles, rhodium is supported onan acidic support, and the NO_(x) absorbent particles are present asparticles separate from the support particles.

[0041]FIG. 4 shows a state where, in the embodiment of FIG. 3, an acidicsupport having supported thereon palladium as the catalyst component isfurther mixed.

[0042] These Figures are only a schematic view for facilitating theunderstanding of the present invention, but the present invention is notlimited thereto.

[0043] In the absorption reduction-type NO_(x) purifying catalyst of thepresent invention, these constituent components are not particularlylimited on the size, however, in terms of the average particle size asan average of longest diameter and shortest diameter (in the case offorming a physically fused aggregate, diameters of the aggregate), thesize is, as a standard, from 0.5 to 5 μm, preferably from 0.5 to 2 μmfor the carrier particle, and from 0.5 to 5 μm, preferably from 0.5 to 2μm for the acidic support particle.

EXAMPLES Example 1

[0044] 20 Parts by mass of an aqueous lanthanum nitrate having aconcentration of 20 mass % as lanthanum oxide was added to 100 parts bymass of zirconium hydroxide powder and mixed. The mixture was dried at80° C. over night and then calcined at 650° C. for 2 hours to obtain aLa_(x)Zr_(1-x)O_((2-x/2)) (x=0.05) powder particles.

[0045] Then, 60 parts by mass of a solution having a concentration of 40mass % as aluminum nitrate, 20 parts by mass of ceria powder and 200parts by mass of water were added to 140 parts by mass of theLa_(x)Zr_(1-x)O_((2-x/2)) (x=0.05) powder prepared above and mixed in aball mill over 8 hours to obtain a slurry.

[0046] This slurry was coated on a monolith substrate and, afterpreliminary calcination by drying, was calcined at 650° C. over one hourto form a layer containing La_(x)Zr_(1-x)O_((2-x/2)) (x=0.05) powderparticles and γ-alumina powder particles on the monolith substrate.

[0047] This layer formed was impregnated with an aqueous dinitrodiammineplatinum solution, then dried and calcined at 500° C. for one hour,thereby loading platinum as the catalyst component.

[0048] Through such a procedure, a catalyst where 3 g of platinum, 100 gof La_(x)Zr_(1-x)O_((2-x/2)) (x=0.05) powder particles and 75 g ofγ-alumina powder particles were supported per 1 liter of the monolithsubstrate was obtained. This catalyst corresponds to the embodiment ofFIG. 1.

Example 2

[0049] A catalyst where 2 g of platinum, 1 g of rhodium, 100 g ofLa_(x)Zr_(1-x)O_((2-x/2)) (x=0.05) powder particles and 75 g ofγ-alumina powder particles were supported per 1 liter of a monolithsubstrate was obtained in the same manner as in Example 1 except thatthe aqueous dinitrodiammine platinum solution of Example 1 was reducedto an amount corresponding to 2 g of platinum and hexaammine rhodiumnitrate was added in an amount corresponding to 1 g of rhodium.

Example 3

[0050] A catalyst where 2 g of platinum, 1 g of palladium, 100 g ofLa_(x)Zr_(1-x)O_((2-x/2)) (x=0.05) powder particles and 75 g ofγ-alumina powder particles were supported per 1 liter of a monolithsubstrate was obtained in the same manner as in Example 2 except thatthe hexaammine rhodium nitrate in an amount corresponding to 1 g ofrhodium of Example 2 was changed to palladium nitrate in an amountcorresponding to 1 g of palladium.

Example 4

[0051] 20 Parts by mass of an aqueous ammonium metatungstate solutionhaving a concentration of 50 mass % was added to 91 parts by mass ofzirconium hydroxide powder and mixed. The mixture was dried at 80° C.over night and then calcined at 650° C. for 2 hours to obtain WO₃/ZrO₂powder particles as an acidic support where tungsten oxide was depositedon zirconium oxide.

[0052] Subsequently, a catalyst where 3 g of platinum, 100 g ofLa_(x)Zr_(1-x)O_((2-x/2)) (x=0.05) powder particles, 75 g of γ-aluminapowder particles and 100 g of WO₃/ZrO₂ powder particles were supportedper 1 liter of a monolith substrate was obtained in the same manner asin Example 1 except that 100 parts by mass of WO₃/ZrO₂ powder particlesprepared above and 75 parts by mass of γ-alumina powder particles wereused in place of 100 parts by mass of γ-alumina powder particle ofExample 1.

Example 5

[0053] A catalyst where 2 g of platinum, 1 g of rhodium, 100 g ofLa_(x)Zr_(1-x)O_((2-x/2)) (x=0.05) powder particles, 75 g of γ-aluminapowder particles and 100 g of WO₃/ZrO₂ powder particles were supportedper 1 liter of a monolith substrate was obtained in the same manner asin Example 4 except that the aqueous dinitrodiammine platinum solutionof Example 4 was reduced to an amount corresponding to 2 g of platinumand hexaammine rhodium nitrate was added in an amount corresponding to 1g of rhodium.

Example 6

[0054] A catalyst where 2 g of platinum, 1 g of palladium, 100 g ofLa_(x)Zr_(1-x)O_((2-x/2)) (x=0.05) powder particles, 75 g of γ-aluminapowder particles and 100 g of WO₃/ZrO₂ powder particles were supportedper 1 liter of a monolith substrate was obtained in the same manner asin Example 5 except that the hexaammine rhodium nitrate in an amountcorresponding to 1 g of rhodium of Example 5 was changed to palladiumnitrate in an amount corresponding to 1 g of palladium.

Example 7

[0055] A catalyst where 2 g of platinum, 0.5 g of rhodium, 0.5 g ofpalladium, 100 g of La_(x)Zr_(1-x)O_((2-x/2)) (x=0.05) powder particles,75 g of γ-alumina powder particles and 100 g of WO₃/ZrO₂ powderparticles were supported per 1 liter of a monolith substrate wasobtained in the same manner as in Example 5 except that the hexaamminerhodium nitrate in an amount corresponding to 1 g of rhodium of Example5 was changed to hexaammine rhodium nitrate in an amount correspondingto 0.5 g of rhodium and palladium nitrate in an amount corresponding to0.5 g of palladium.

Comparative Example 1

[0056] In Example 1, a layer containing γ-alumina powder particles wasformed on a monolith substrate without containingLa_(x)Zr_(1-x)O_((2-x/2)) (x=0.05) powder particles. This layer wasimpregnated with an aqueous dinitrodiammine platinum solution, thendried and calcined at 500° C. for one hour, thereby loading platinum asthe catalyst component.

[0057] Subsequently, the layer was further impregnated with an aqueousbarium acetate solution and an aqueous potassium acetate solution, thendried and calcined at 500° C. for on hour to obtain a catalyst where 3 gof platinum, 0.2 mol of barium, 0.1 mol of potassium and 120 g ofγ-alumina powder particle were supported per 1 liter of the monolithsubstrate. This catalyst corresponds to the embodiment of FIG. 2.

[0058] —NO_(x) Absorption Percentage Test—

[0059] The catalyst obtained in Examples 1 to 7 and Comparative Example1 each was measured on the NO_(x) absorption percentage immediatelyafter the preparation under the following conditions. The resultsobtained are shown in Table 1.

[0060] Exhaust gas: A/F=22

[0061] Exposure time: 1 minute

[0062] Gas space velocity: 50,000 h⁻¹ TABLE 1 NO_(x) AbsorptionPercentage at Preparation (for 1 minute in lean time) Noble Metal NO_(x)2 g of Pt + Acidic Absorption Percentage (%) 1 g of Each Support 200° C.300° C. 400° C. Example 1 Pt none 56.5 96.7 86.4 Example 2 Rh none 58.298.1 86.9 Example 3 Pd none 86.5 96.7 86.7 Example 4 Pt WO₃/ZrO₂ 58.196.2 77.0 Example 5 Rh WO₃/ZrO₂ 60.1 98.2 79.1 Example 6 Pd WO₃/ZrO₂85.4 97.5 80.5 Example 7 Rh/Pd WO₃/ZrO₂ 88.2 98.1 81.5 Comparative Ptnone 41.6 91.6 98.4 Example 1

[0063] Furthermore, each catalyst was subjected to an endurancetreatment under the following conditions and measured for the NO_(x)absorption percentage after the endurance treatment in the same manner.The results obtained are shown in Table 2.

[0064] Exhaust gas: A/F was varied between 14 and 20 in a cycle of 30seconds

[0065] Exhaust gas temperature: 850° C.

[0066] Exposure time: 100 hours

[0067] Gas space velocity: 100,000 h⁻¹ TABLE 2 NO_(x) AbsorptionPercentage after Endurance Treatment (for 1 minute in lean time) NobleMetal NO_(x) 2 g of Pt + Acidic Absorption Percentage (%) 1 g of EachSupport 200° C. 300° C. 400° C. Example 1 Pt none 47.1 86.4 63.5 Example2 Rh none 40.1 78.4 64.3 Example 3 Pd none 35.6 76.5 64.2 Example 4 PtWO₃/ZrO₂ 54.1 92.3 67.4 Example 5 Rh WO₃/ZrO₂ 53.1 93.5 70.1 Example 6Pd WO₃/ZrO₂ 79.1 94.5 74.5 Example 7 Rh/Pd WO₃/ZrO₂ 95.2 96.6 78.0Comparative Pr none 15.2 78.1 54.1 Example 1

[0068] It is seen from the results shown in Table 1 that when theabsorbent comprising an alkali metal or alkaline earth metal is replacedby an absorbent comprising La_(x)Zr_(1-x)O_((2-x/2)) powder particles(Examples 1 to 3), the NO_(x) absorption percentage, particularly at lowtemperature of 200° C., is increased. Furthermore, when WO₃/ZrO₂ powderparticles which are an acidic support is contained (Examples 4 to 7),the NO_(x) absorption percentage is more increased.

[0069] Also, it is seen from the results shown in Table 2 that theimprovement of the NO_(x) absorption percentage at low temperatures ismaintained even after the endurance heat treatment as compared withconventional catalysts and this effect is more enhanced by containing anacidic support.

[0070] —NO Oxidizing Capability Test—

[0071] The catalysts of Examples 2 and 5 and Comparative Example 1 wereeach exposed to an exhaust gas at 800° C. having a theoretical air-fuelratio (A/F=14) for 50 hours. Thereafter, an exhaust gas was introducedunder the following conditions and the percentage of NO oxidized intoNO_(x) was measured by varying the temperature of exhaust gas enteringinto the catalyst. FIG. 5 shows the results.

[0072] Gas composition: 250 ppm of NO+6% of O₂+10% of CO₂+8% of H₂O(balance: nitrogen)

[0073] Gas space velocity: 50,000 h⁻¹

[0074] It is seen from the results shown in FIG. 5 that the catalyst ofthe present invention is greatly increased in the NO oxidizationpercentage at low temperatures. This increase of the NO oxidationpercentage is understood to correspond to the increase of the NO_(x)absorption percentage in Table 1.

[0075] The reason therefor is considered as follows. The comparativecatalyst is suppressed in the catalytic activity because the NO_(x)absorbing component and the catalyst component are supported on the samesupport, whereas in the present invention, the NO_(x) absorbingcomponent is separated from the support having thereon the catalystcomponent and therefore, the catalytic activity is not suppressed.

[0076] —SO_(x) Desorption Test—

[0077] The catalysts of Examples 2 and 5 and Comparative Example 1 eachwas subjected to a sulfur poisoning treatment by elevating thetemperature to 250 to 550° C. for 30 minutes in an atmosphere of gashaving the following composition.

[0078] Gas composition: 100 ppm of SO₂+150 ppm of CO+670 ppm of C₃H₆+250ppm of NO+10% of O_(2+6.5)% of CO₂+3% of H₂O (balance: nitrogen)

[0079] Gas space velocity: 100,000 h⁻¹

[0080] Then, the catalyst having adsorbed thereto SO_(x) by thistreatment was heated to a temperature between 150° C. and 750° C. at 20°C./min in an exhaust gas atmosphere of A/F=14 and the concentration ofSO_(x) desorbed was measured. FIG. 6 shows the results.

[0081] It is seen from the results shown in FIG. 6 that, in the catalystof the present invention, the SO_(x) desorbing temperature was greatlyshifted to the low-temperature side.

[0082] The reason therefor is considered to be as follows. In theconventional catalyst, the NO_(x) absorbing component and the catalystcomponent are supported on the same support and the activity of thecatalyst component is thereby suppressed. Furthermore, potassium andbarium having high reactivity of forming a nitrate are used as theNO_(x) absorbent and therefore, the formation of a sulfate proceedsinside the NO_(x) absorbent, as a result, the SO_(x) desorption isworsened.

[0083] On the other hand, in the present invention, the catalystcomponent is not suppressed in the activity because the NO_(x) absorbingcomponent is separated from the support having thereon the catalystcomponent, and moreover, the bonding between SO_(x) and the NO_(x)absorbent is relatively weak.

[0084] It is also seen that, in the case of adding an acidic support(Example 5) to the support, the activity of the acidic supportcontributes to the SO_(x) desorption in the low-temperature side.

[0085] This is considered to be because an acidic condition is formedfor the catalyst component, and NO_(x) and SO_(x) move quickly on anacidic support.

INDUSTRIAL APPLICABILITY

[0086] As described in the foregoing pages, the catalyst of the presentinvention is enhanced in the NO_(x) purifying capability at lowtemperatures and improved in the SO_(x) desorbing property. Accordingly,a catalyst enlarged in the temperature range where a high three-wayperformance can be exerted, and suitable for the purification of adiesel exhaust gas, can be provided.

1. An absorption reduction-type NO_(x) purifying catalyst where NO_(x)absorbent particles and support particles having supported thereon acatalyst component are mixed.
 2. The absorption reduction-type NO_(x)purifying catalyst as claimed in claim 1, wherein acidic supportparticles are added to said support particle.
 3. The absorptionreduction-type NO_(x) purifying catalyst as claimed in claim 1 or 2,wherein said NO_(x) absorbent particles are a metal oxide having a basepoint.
 4. The absorption reduction-type NO_(x) purifying catalyst asclaimed in claim 3, wherein said metal oxide is rare earth-addedzirconia or alkaline earth-added zirconia.